1. Field of the Invention
This invention relates to an optical displacement measurement system for detecting the relative movement, if any, of a movable part of a semiconductor manufacturing apparatus, a machine tool or some other apparatus.
2. Description of Related Art
Optical displacement measurement systems utilizing a diffraction grating to detect the relative movement of a movable part of an apparatus such as a semiconductor manufacturing apparatus or a machine tool are known.
For example, FIGS. 1 and 2 of the accompanying drawings show a known optical displacement measurement system described in Japanese Patent Application Laid-Open No. 60-98302. FIG. 1 is a schematic perspective view of the known optical displacement measurement system 100 and FIG. 2 is a schematic view of the optical displacement measurement system 100 as viewed along arrow N1 in FIG. 1.
This known optical displacement measurement system 100 comprises a diffraction grating 101 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a coherent light source 102 for emitting a coherent laser beam, a half mirror 103 for dividing the laser beam emitted from the coherent light source 102 into two beams and causing the two diffracted beams from the diffraction grating 101 to overlap and interfere with each other, a pair of mirrors 104a, 104b for reflecting the respective beams diffracted by the diffraction grating 101 and a photodetector 105 for receiving the two diffracted beams and generating an interference signal.
The laser beam emitted from the coherent light source 102 is split into two beams by the half mirror 103. Then, the two beams are made to strike the diffraction grating 101. The two beams striking the diffraction grating 101 are then diffracted by the diffraction grating 101 and leave the latter as diffracted beams. The two primary diffracted beams diffracted by the diffraction grating 101 are subsequently reflected by the mirrors 104a, 104b respectively. The diffracted beams reflected by the respective mirrors 104a, 104b are made to strike the diffraction grating 101 once again and diffracted by the diffraction grating 101 for another time before being returned to the half mirror 103, reversely following the same light paths. The diffracted beams returned to the half mirror 103 are caused to overlap and interfere with each other before being detected by the photodetector 105.
With the known optical displacement measurement system 100, the diffraction grating 101 moves in directions indicated by arrows X1, X2 respectively. Then, in the optical displacement measurement system 100, the two diffracted beams produced by the diffraction grating 101 show a phase difference as a function of the movement of the diffraction grating 101. Thus, the optical displacement measurement system 101 can determine the displacement of the movable part of the machine tool by detecting the phase difference of the two diffracted beams from the interference signal produced by the photodetector 105.
FIGS. 3 and 4 of the accompanying drawings show another known optical displacement measurement system described in Japanese Patent Application Laid-Open No. 60-98302. FIG. 3 is a schematic perspective view of the known optical displacement measurement system 110 and FIG. 4 is a schematic view of the optical displacement measurement system 110 as viewed along arrow N1 in FIG. 3.
This known optical displacement measurement system 110 comprises a diffraction grating 111 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a coherent light source 112 for emitting a coherent laser beam, a half mirror 113 for dividing the laser beam emitted from the coherent light source 112 into two beams and causing the two diffracted beams from the diffraction grating 111 to overlap and interfere with each other, a first pair of mirrors 114a, 114b for reflecting the respective beams diffracted by the diffraction grating 111 to a same and identical spot on the diffraction grating 111 and a second pair of mirrors 115a, 115b for reflecting the respective diffracted beams diffracted by the diffraction grating 111 and a photodetector 116 for receiving the two diffracted beams and generating an interference signal.
The laser beam emitted from the coherent light source 112 is split into two beams by the half mirror 113. Then, the two beams are reflected respectively by the first pair of mirrors 114a, 114b and made to strike the diffraction grating 111 as a same and identical spot. The two beams striking the diffraction grating 111 are then diffracted by the diffraction grating 111 and leave the latter as diffracted beams. The two primary diffracted beams diffracted by the diffraction grating 111 are subsequently reflected by the second pair of mirrors 115a, 115b respectively. The diffracted beams reflected by the second pair of mirrors 104a, 104b are made to strike the diffraction grating 111 once again and diffracted by the diffraction grating 111 for another time before being returned to the half mirror 113, reversely following the same light paths. The diffracted beams returned to the half mirror 113 are caused to overlap and interfere with each other before being detected by the photodetector 116.
With the known optical displacement measurement system 110, the diffraction grating 111 moves in directions indicated by arrows X1, X2 respectively. Then, in the optical displacement measurement system 110, the two diffracted beams produced by the diffraction grating 111 show a phase difference as a function of the movement of the diffraction grating 111. Thus, the optical displacement measurement system 111 can determine the displacement of the movable part of the machine tool by detecting the phase difference of the two diffracted beams from the interference signal produced by the photodetector 116.
Now, with the trend of enhanced high precision of machine tools and industrial robots in recent years, optical displacement measurement systems of the type under consideration are required more often than not to have a position detecting capability with a degree of resolution of tens of several nanometers to several nanometers.
For an optical displacement measurement system to have a high degree of resolution, it is required to detect a large interference signal. Then, the two diffracted beams to be made to interfere with each other have to be overlapped with a very high degree of precision.
However, with either of the above described known optical displacement measurement systems 100, 110, the diffracted beams can become displaced from each other to abruptly dwarf the interference signal and make it impossible to detect the position of the movable part if the diffraction grating 101 or 111, whichever appropriate, is moved in a direction other than the right direction of movement or has undulations. For example, if the diffraction grating 101 or 111 is rotated in the directions of arrows A1 and A2 of B1 and B2 as shown in FIGS. 1 through 4, it is no longer possible to detect the position of the movable part or the machine tool that is under scrutiny.
FIG. 5 of the accompanying drawings shows an optical displacement measurement system 120 obtained by modifying the above described known optical displacement measurement system 100. Referring to FIG. 5, it has a first lens 106 for focussing the laser beams emitted from the coherent light source 102 on the mirrors 104a, 104b and a second lens 107 for focussing the two diffracted beams that have been made to overlap and interfere with each other by the half mirror 103 on the light receiving plane of the photodetector 105.
However, this optical displacement measurement system 120 is also not free from the above pointed out problem that the diffracted beams can become displaced from each other to abruptly dwarf the interference signal and make it impossible to detect the position of the movable part if the diffraction grating 101 is moved in a direction other than the right direction of movement or has undulations.
For instance, if the diffraction grating 101 is angularly moved by about {fraction (1/60)} of a degree in the directions of arrows A1 and A2 and about ⅙ of a degree in the directions of arrows B1 and B2, the magnitude of the interference signal will change by 20%. If a reflection type diffraction grating is used, the angle of tolerance in the directions of arrow B1 and B2 will be reduced to a fraction of the above cited value to make it further difficult to detect the position of the movable part.
FIG. 6 of the accompanying drawings illustrates a known optical displacement measurement system described in Japanese Patent Application Laid-Open No. 2-167427.
Referring to FIG. 6, the optical displacement measurement system 130 comprises a diffraction grating 131 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a laser diode 132 for emitting a laser beam, a first half mirror 133 for dividing the laser beam emitted from the laser diode 132, first and second light receiving elements 134, 135 for receiving the two diffracted beams transmitted through the diffraction grating 131, a pair of lenses 136, 137 for focussing the two diffracted beams respectively and a second half mirror 138 for separating and synthetically combining the two diffracted beams focussed by the pair of lenses 136, 137.
The optical displacement measurement system 130 further comprises a first pair of mirrors 139, 140 for reflecting the laser beams produced by the half mirror 133 and causing them to strike the diffraction grating 131, a second pair of mirrors 141, 142 for reflecting the laser beams transmitted by the diffraction grating 131 and causing them to strike the half mirror 138, a xc2xc wave plate 143 and a first analyser 144 arranged between the first light receiving element 143 and the half mirror 138 and a second analyser 145 arranged between the second light receiving element 135 and the half mirror 138.
In the optical displacement measurement system 130, the first and second lenses 136, 137 are arranged in such a way that they focus respective beams on the diffraction plane or the refraction plane of the diffraction grating 131. Therefore, the diffracted beams respectively striking the first and second light receiving elements are always held in parallel with each other and the interference signal will fluctuate little if the diffraction grating 131 shows undulations.
However, the proposed optical displacement measurement system 130 only ensures the parallelism of the two diffracted beams. That is, if the diffraction grating 131 is inclined, a uniform interference will be maintained only in the shaded area in FIG. 7 where the two beams are made to overlap with each other. In other words, the two diffracted beams do not interfere with each other in areas other than the area where the two beams are made to overlap with each other so that consequently the obtained interference signal will become dwarfed. Additionally, if the two beams are not strictly parallel relative to each other and involve aberration in any sense of the word, no uniform interference will be ensured even in the area where the two beams are made to overlap with each other.
FIG. 8 of the accompanying drawings illustrates a known optical displacement measurement system described in Japanese Patent Application Laid-Open No 1-185415.
Referring to FIG. 8, the known optical displacement measurement system 150 comprises a transmission type diffraction grating 151 adapted to linearly move in directions indicated respectively by arrows X1 and/or X2 in the drawings in response to a movement of the movable part of a machine tool, a laser diode 152 for emitting a laser beam, a collimator lens 153 for collimating the laser beam emitted from the laser diode 152, a first half mirror 154 for dividing the collimated laser beam into two beams, a first pair of mirrors 155a, 155b for respectively reflecting the divided beams and cause them to strike the diffraction grating 151, a second pair of mirrors 156a, 156b for respectively reflecting the diffracted beams produced by the diffraction grating 151 as the divided beams are transmitted therethrough, a pair of polarizers 157a, 157b for causing the diffracted beams reflected by the second pair of mirrors 156a, 156b to intersect each other rectangularly, a second half mirror 158 for causing the two diffracted beams to overlap with each other, a first light receiving element 159 for receiving the two diffracted beams made to overlap with each other by the second half mirror 158, a third half mirror 160 for separating the diffracted beams made to overlap with each other by the second half mirror 158, second and third light receiving elements 161 and 162 for respectively receiving the beams produced by the third half mirror 160, an analyser 163 arranged between the third half mirror 160 and the second light receiving element 161 and a xc2xc wave plate 164 and another analyser 165 arranged between the third half mirror 160 and the third light receiving element 162.
The two coherent beams of light produced by the first half mirror 154 by dividing the original coherent light beam is regulated for the incident angles respectively by the first pair of mirrors 155a, 155b so that they are made equal to xcex8. The two coherent beams are made to strike the lattice plane of the diffraction grating 151 at a same and identical spot. The diffracted beams produced from the coherent beams striking the lattice plane with the angle of incidence of xcex8 shows a same angle of diffraction of "PHgr". With this optical displacement measurement system 150, beams of the 0-th degree do not stray into the light paths of the diffracted beams because the angle of incidence and that of diffraction are differentiated. Therefore, no noise would be generated by a beam of the 0-th degree to make the system capable of reliably detecting the position of the movable part.
However, two coherent beams of light are made to strike the lattice plane of the diffraction grating 151 at a same spot with a same angle of incidence in the above optical displacement measurement system 150. Then, as seen from FIG. 9, the reflected beam produced when one of the coherent beams strikes the diffraction grating 151 travels backwardly the path of the other coherent beam striking the diffraction grating 151 and consequently enters the laser diode 152.
Generally, a laser diode is highly sensitive to a returning beam and made unstable in terms of oscillation and noise generation by such a beam. Then, the wavelength of the laser beam emitted from the laser diode will become unstable. The S/N ratio and the stability of the interference signal will be severely damaged as a reflected beam returns to the laser diode 152 of the optical displacement measurement system 150.
Therefore, it is an object of the present invention to provide an optical displacement measurement system that can detect the position of a movable part of a machine tool with an enhanced degree of resolution.
Another object of the present invention is to provide an optical displacement measurement system with which any beam reflected by the diffraction grating thereof does not return to the light emitting means of the system so that the position of a movable part of a machine tool can be detected reliably with an enhanced degree of resolution.
According to the invention, the above objects and other objects of the invention are achieved by providing an optical displacement measurement system characterized by comprising a diffraction grating adapted to be irradiated with a coherent beam of light and move in directions parallel to the lattice vector relative to the coherent beam to diffract the coherent beam, a light emitting means for emitting a coherent beam of light, an irradiation optical system for dividing the coherent beam of light emitted from said light emitting means into two coherent beams of light and irradiating said diffraction grating with each of the coherent beams, an interference optical system for causing each of the coherent beams to make the two diffracted beams of light obtained by the diffracting operation of said diffraction grating interfere with each other, a light receiving means for receiving the two diffracted beams interfering with each other and detecting an interference signal and a position detecting means for determining the phase difference of the two diffracted beams from the interference signal detected by said light receiving means and detecting the position of the relatively moved diffraction grating, said irradiation optical system having a first focussing means for focussing the two coherent beams irradiating the diffraction grating on the lattice plane of the diffraction grating, said interference optical system having a second focussing means for focussing the two diffracted beams interfering with each other and received by said light receiving means on the light receiving plane of the light receiving means.
With an optical displacement measurement system having a configuration as described above and schematically illustrated in FIG. 10, the first focussing means 4 focusses the coherent beam of light La emitted from the light emitting means 2 on the lattice plane of the diffraction grating 1. Then, the coherent beam La focussed on the lattice plane of the diffraction grating 1 is diffracted by the diffraction grating 1 to produce a diffracted beam Lb as a result of reflection or transmission. Then, the second focussing means 5 focusses the diffracted beam Lb on the light receiving plane of the light receiving means 3.
With an optical displacement measurement system having a configuration as described above, the length of the light path travelled by the diffracted beam laser beam passing through the aperture of the second focussing means 5 is invariably held to a constant value because the coherent beam La is focussed on the lattice plane of the diffraction grating 1 while the diffracted beam Lb is focussed on the light receiving plane of the light receiving means 3. Therefore, the focussing position of the light receiving plane of the light receiving means 3 does not vary and the length of the light path travelled by the diffracted beam is invariably held to a constant value if the optical axis of the diffracted beam Lb is shifted for some reason or other.
According to another aspect of the invention, there is also provided an optical displacement measurement system characterized by comprising a diffraction grating adapted to be irradiated with a coherent beam of light and move in directions parallel to the lattice vector relative to the coherent beam to diffract the coherent beam, a light emitting means for emitting a coherent beam of light, an irradiation optical system for dividing the coherent beam of light emitted from said light emitting means into two coherent beams of light and irradiating said diffraction grating with each of the coherent beams, an interference optical system for causing each of the coherent beams to make the two diffracted beams of light obtained by the diffracting operation of said diffraction grating interfere with each other, a light receiving means for receiving the two diffracted beams interfering with each other and detecting an interference signal and a position detecting means for determining the phase difference of the two diffracted beams from the interference signal detected by said light receiving means and detecting the position of the relatively moved diffraction grating, said irradiation optical system being adapted to form optical paths respectively for said two coherent beams on a plane inclined relative to the direction perpendicular to the lattice plane of said diffraction grating and irradiate a same and identical spot on the lattice plane of said diffraction grating with the two coherent beams.
With an optical displacement measurement system having a configuration as described above, optical paths are formed in a direction inclined relative to the direction perpendicular to the lattice plane of the diffraction grating respectively for the two coherent beams and the coherent beams are made to irradiate a same and identical spot on the lattice plane of the diffraction grating. Then, the phase difference of the two diffracted beams produced by the two coherent beams is determined to detect the relative displacement of the diffraction grating.